Recently, reduction in cost and increase in the processing speed in devices handling color picture data are progressing speedily and, in keeping pace therewith, a wide variety of systems, handling color pictures, exemplified by a system exchanging color picture data over a network, such as the Internet, or a desk top publishing system for carrying out the editing operations, including color picture data, with the aid of a computer, are rapidly coming into extensive use.
The devices handling color pictures differ in input/output characteristics of expressible colors or gradations, depending on the device sorts, such that, if the color picture data are simply exchanged between the devices of different sorts, the colors reproduced become different from one device to another. For example, if, when a picture displayed on a monitor is output as a hard copy on a printer, the color gamut that may be represented on a monitor differs from the gamut that may be represented on a printer, it may be an occurrence that the color of a picture represented on the monitor differs from that of a picture output as a hard copy by the printer.
As a large variety of systems handling color pictures have been put to practical use, the concept of a so-called device independent color in which color pictures of the same colors may be reproduced between different sorts of the devices on the system has become crucial. The system which implements the device independent color is generally termed a color management system, a representative example of which is Colorsync of MacOS and ICM of Windows.
In the color management system, the device independent color is implemented by matching physical calorimetric values of the color signals of the input/output device. Specifically, color signals of an input picture from an input device, such as a video camera 61, a scanner 62 and a monitor 63, are converted into color signals in a device-independent color space (e.g. CIE/XYZ, CIE/L*a*b*) based on a device profile the color gamut conversion equation or the color gamut conversion table of which has been defined from one device to another, as shown in FIG. 1. When the color signals are output from output devices, such as a monitor 63 or a printer 64, these color signals are converted into output color picture signals in the color space associated with the devices based on a device profile having a color gamut changing equation or a color gamut conversion table defined from one device to another.
Thus, if, with the color management system, color signals are converted from an input picture color signal of an input system to an output color picture signal of an output system, the input picture color signal is converted once into color signals in a device-independent color space, based on the device profile, for realization of the device-independent color, as shown in FIG. 2. Meanwhile, the device profile is a file in which a color gamut changing equation or a color gamut conversion table has been defined. Stated differently, the device profile is a file in which there are stored a set of parameters calculated from the relation between the color signal values of the device (e.g. RGB or CMYK) and color values as measured by e.g. a colorimeter (e.g. XYZ or L*a:b*).
In case the relation between color signal values of a device and color values of a picture realized by these color signal values is non-linear, as in a printer, the routine practice is to use a lookup table (LUT) as a device profile.
Color printing in a color printer is realized by controlling the amount of deposition of colorants of cyan (c), magenta (m) and yellow (y) with black (k). However, in a control interface for each of specified printer types, color signals for printing received in particular by machine types of general usage or application are mostly not cmy(k) but RGB signals. Since picture input equipment, such as digital cameras, output equipment, such as display monitors, or the application software, usually handle colors represented in RGB, it is naturally convenient to issue commands with RGB for printing as well. It should be noted that the control interface means a so-called printer driver software, operating on a calculating device, such as a personal computer, distinct from the printer, a firmware operating on a calculating device mounted within the main body unit of the printer, or an electronic circuit within the inside of the main body unit of the printer, and denotes a processor for converting input color signals for printing into control signals for a specified printer. Inherently, RGB is device signals, such as signals of monitors or cameras. Recently, as standard encoding in the sRGB color space [IEC 61966-2.1] has come into extensive use, control interfaces of printers of different types receiving these data are now able to make substantially common interpretation. However, the color gamut reproducible with a printer is locally wider than the color gamut limited with the encoding of the sRGB color space, such that, if a printer receives signals encoded with sRGB, the color reproducing capabilities of the printer cannot be exploited sufficiently.
On the other hand, in the DCF Version 1.0 [JEIDA49-2-1998], extensively used in Japan, there is shown a guideline for encoding picture data with the YCC (luminance chromaticity separation color space) uniquely associated with the sRGB color space. If, in the YCC not restricted to the color gamut of sRGB, the camera has also recorded data outside the sRGB color gamut, and if, in effecting color conversion of the recorded YCC for printer outputting, picture data is encoded once in the sRGB color space, data recorded by the camera cannot be exploited sufficiently. Thus, in a printer having an interface for directly reading picture data, imaged by e.g. a digital still camera, from a camera or a recording medium, attempts are made to effect color conversion of data, recorded with YCC, based on a unique interpretation which expands the color gamut such as by allowing negative values in the sRGB color space in the processing inside the printer. However, when imaged picture data of a camera are sent to a printer driver on a personal computer via e.g. an application software, it is necessary for the application software and the printer driver to make common interpretation as to the expansion of the color gamut. Thus, it is efficacious to use the standard color space which expands the color gamut of the sRGB color space to enable the encoding, such as sYCC, for example, IEC61966-2.1 AnnexF, the standardization for which is currently proceeding in IEC, as in Exit2.2 standard picture file format standard Version 2.2 (for a digital still camera).
However, if the color gamut that can be encoded is expanded to add the information outside the color gamut, the information volume inside the color gamut is relatively diminished. If signals of three channels is encoded by sRGB and sYCC with the same data volume, the sYCC is lower in density of the visual information in a portion of the perception-equal color space close to the achromatic color contained in a larger quantity in a natural picture. For showing this example, FIG. 3 plots the gamut of L*=50±5 of values encoded with the information volume for both sRGB/sYCC on a chromaticity diagram of the CIELAB color space. This leads to disruption of gray balance, pseudo-contour and collapse in gradation to deteriorate the picture quality. It is noted that the information of the 4 [bit/channel] is formed as the values encoded with 8 [bit/channel] is uniformly decimated based on sRGB/sYCC.
Meanwhile, in the color management system, the device profile, used to match the colorimetric values among different devices, states the device color reproducing characteristics in terms of the relation between device signal values (e.g. RGB or cymk) and color values (e.g. XYZ or L*a*b*). In the case of a device, such as a printer, this relation is generally expressed in the form of a lookup table.
On the other hand, if a printer control interface converts data for printing of input signals (e.g. RGB or cymk) into printer control signals (e.g. cymk) in e.g. a printer driver software operating on e.g. a personal computer, an appropriate LUT, prepared from the outset in consideration of color reproducibility of a target printer, is referenced to perform color space conversion calculations. Since the LUT influences the color reproducibility, such a LUT having a large number of lattice points and thus having a higher density is preferably used in order to obtain high precision color reproducing results. If the storage capacity of a recording medium is taken into account, it is required to prepare the LUT with a limited number of lattice points. In the above-described method for preparing the LUT, grid positions are calculated by equal division even in the chromaticity directions, such as a* or b*, the number of grid points of the LUT entering the device color gamut is small, and hence the amount of the information correlating the device signals and the color signals in the color gamut is small, with the result that the color reproducibility is not that good.
In order to overcome such deficiency, the present Assignee has proposed, in the specification and drawings of the Japanese Patent Application 2000-340456, a color correction processing method in which the grids of the LUT are not of uniform intervals and non-linear positions obtained on applying a suitable S-function are used to diminish the grid density in the vicinity of the gray area.
It is an object of the present invention to provide a picture signal processing method and a picture signal processing apparatus in which this technique for enhancing the efficiency is applied not to the LUT referenced by the conversion processing system for the printing data but to the printing data itself to render it possible to locally control the encoding density in the broad color gamut to improve the color reproducing quality.